I want to remove said directory path & filename and end up with just INVALID ORDER-NO 4546090.

Two approaches. One might fit for you.

Approach 1 - nuke leading paths   s#^/[^ ] //; Approach 2 - isolate only the part you want 

  m/(INVALID ORDER-NO \d+)/;
  $_ = $1;

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In reply to Re: A poor bloke and his regex... by dws
in thread A poor bloke and his regex... by vek

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